Merck Manual

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Overview of Vitamins

By

Larry E. Johnson

, MD, PhD, University of Arkansas for Medical Sciences

Last full review/revision Oct 2019| Content last modified Oct 2019
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Topic Resources

Vitamins are a vital part of a healthy diet. The recommended dietary allowance (RDA)—the amount most healthy people need each day to remain healthy—has been determined for most vitamins. A safe upper limit (tolerable upper intake level) has been determined for some vitamins. Intake above this limit increases the risk of a harmful effect (toxicity).

Did You Know...

  • Consuming very large doses of certain vitamins can be harmful.

Consuming too little of a vitamin can cause a nutritional disorder. However, people who eat a variety of foods are unlikely to develop most vitamin deficiencies. Deficiency of vitamin D is an exception. Vitamin D deficiency is common among certain groups of people (such as older people) even if they eat a variety of foods. For other vitamins, a deficiency can develop if people follow a restrictive diet that does not contain enough of a particular vitamin. For example, vegans, who consume no animal products, may become deficient in vitamin B12, which is available in animal products. Deficiency of the B vitamins biotin or pantothenic acid almost never occurs.

Consuming large amounts (megadoses) of certain vitamins (usually as supplements) without medical supervision may also have harmful effects.

Vitamins are called essential micronutrients because the body requires them but only in small amounts.

The body does not store most vitamins. Deficiencies of these vitamins usually develop in weeks to months. Therefore, people must consume them regularly.

Vitamins A, B12, and D are stored in significant amounts, mainly in the liver. Vitamins A and D are also stored in fat cells. Deficiencies of these vitamins take more than a year to develop.

Because many people eat irregularly or do not eat a variety of foods, they may not get enough of some vitamins from foods alone. If they do not get enough, the risk of certain cancers or other disorders may be increased. People may then take a multivitamin. However, for most people, taking multivitamins does not appear to reduce risk of developing cancer or heart or blood vessel (cardiovascular) disorders.

Table
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Vitamins

Vitamin

Good Sources

Main Functions

Recommended Dietary Allowance for Adults

Safe Upper Limit

Biotin

Liver, kidneys, meats, eggs, milk, fish, dried yeast, sweet potatoes, seeds, and nuts

Required for the processing (metabolism) of carbohydrates and fatty acids

30 micrograms (but no RDA has been established)

35 micrograms for breastfeeding women

Folate (folic acid)

Raw green leafy vegetables, asparagus, broccoli, fruits (especially citrus), liver, other organ meats, dried yeast, and enriched breads, pastas, and cereals

(Note: Extensive cooking destroys 50–95% of the folate in food.)

Required for the formation of red blood cells, for DNA and RNA synthesis, and for normal development of the nervous system in a fetus

400 micrograms

600 micrograms for pregnant women

500 micrograms for breastfeeding women

1,000 micrograms

Niacin (nicotinic acid or nicotinamide)

Dried yeast, liver, red meat, poultry, fish, legumes, and whole-grain or enriched cereal products and bread

Required for the metabolism of carbohydrates, fats, and many other substances and for the normal functioning of cells

14 milligrams for women

16 milligrams for men

18 milligrams for pregnant women

17 milligrams for breastfeeding women

35 milligrams

Pantothenic acid

Liver, beef, egg yolks, yeast, potatoes, broccoli, and whole grains

Required for the metabolism of carbohydrates and fats

5 milligrams (but no RDA has been established)

6 milligrams for pregnant women

7 milligrams for breastfeeding women

Riboflavin (vitamin B2)

Milk, cheese, liver, meat, fish, eggs, and enriched cereals

Required for the metabolism of carbohydrates and proteins and for healthy mucous membranes, such as those lining the mouth

1.1 milligrams for women

1.3 milligrams for men

1.4 milligrams for pregnant women

1.6 milligrams for breastfeeding women

Thiamin (vitamin B1)

Dried yeast, whole grains, meat (especially pork and liver), enriched cereals, nuts, legumes, and potatoes

Required for the metabolism of carbohydrates, proteins, and fats and for normal nerve and heart function

1.1 milligrams for women

1.2 milligrams for men

1.4 milligrams for pregnant or breastfeeding women

Vitamin A (retinol)

As vitamin A: Fish liver oils, liver, egg yolks, butter, cream, and fortified milk

As carotenoids (converted to vitamin A in the body), such as beta-carotene: Dark green, yellow, and orange vegetables and yellow and orange fruits

Required to form light-sensitive nerve cells (photoreceptors) in the retina, helping maintain night vision

Helps maintain the health of the skin, cornea, and lining of the lungs, intestine, and urinary tract

Helps protect against infections

700 micrograms for women

900 micrograms for men

770 micrograms for pregnant women

1,300 micrograms for breastfeeding women

3,000 micrograms

Vitamin B6 (pyridoxine)

Dried yeast, liver, other organ meats, whole-grain cereals, fish, and legumes

Required for the metabolism of carbohydrates, amino acids, and fats, for normal nerve function, for the formation of red blood cells, and for healthy skin

1.3 milligrams for younger women and men

1.5 milligrams for women older than 50

1.7 milligrams for men older than 50

1.9 milligrams for pregnant women

2.0 milligrams for breastfeeding women

100 milligrams

Vitamin B12 (cobalamins)

Meats (especially beef, pork, liver, and other organ meats), eggs, fortified cereals, milk, clams, oysters, salmon, and tuna

Required for the formation and maturation of red blood cells, for nerve function, and for DNA synthesis

2.4 micrograms

2.6 micrograms for pregnant women

2.8 micrograms for breastfeeding women

Vitamin C (ascorbic acid)

Citrus fruits, tomatoes, potatoes, broccoli, strawberries, and sweet peppers

Required for the formation, growth, and repair of bone, skin, and connective tissue; for healing of wounds and burns; and for normal function of blood vessels

Acts as an antioxidant, protecting cells against damage by free radicals

Helps the body absorb iron

75 milligrams for women

90 milligrams for men

85 milligrams for pregnant women

120 milligrams for breastfeeding women

35 milligrams more for smokers

2,000 milligrams

Formed in the skin when the skin is exposed to direct sunlight

Fortified milk and dairy products, fatty fish, fish liver oils, liver, and egg yolks

Promotes the absorption of calcium and phosphorus from the intestine

Required for bone formation, growth, and repair

Strengthens the immune system and reduces the risk of autoimmune disorders

600 units for people aged 1‒70

800 units for people older than 70

4,000 units

Vegetable oil, nuts, seeds, green leafy vegetables, and wheat germ

Acts as an antioxidant, protecting cells against damage by free radicals

15 milligrams (22 units of natural or 33 units of synthetic)

19 milligrams for breastfeeding women

1,000 milligrams

Green leafy vegetables (such as collards, spinach, and kale) and soybean and canola oils

Helps in the formation of blood clotting factors and thus is necessary for normal blood clotting

Required for healthy bones and other tissues

90 micrograms for women

120 micrograms for men

DNA = deoxyribonucleic acid; RNA = ribonucleic acid; RDA=recommended daily allowance.

Some vitamins are fat soluble. Other vitamins are water soluble. The difference between fat and water soluble affects nutrition in several ways.

Fat-soluble vitamins

Fat-soluble vitamins dissolve in fats (lipids) and include

  • Vitamin A

  • Vitamin D

  • Vitamin E

  • Vitamin K

Fat-soluble vitamins are stored in the liver and in fatty tissues. If too much of the fat-soluble vitamins A or D are consumed, they can accumulate and may have harmful effects.

Because fats in foods help the body absorb fat-soluble vitamins, a low-fat diet may result in a deficiency. Some disorders interfere with absorption of fats and thus of fat-soluble vitamins. These are called malabsorption disorders. Examples are chronic diarrhea, Crohn disease, cystic fibrosis, certain pancreatic disorders, and blockage of the bile ducts. Some drugs, such as mineral oil, have the same effect. Fat-soluble vitamins dissolve in mineral oil, which the body does not absorb. So when people take mineral oil (for example, as a laxative), it carries these vitamins unabsorbed out of the body.

Cooking does not destroy fat-soluble vitamins.

Water-soluble vitamins

Water-soluble vitamins dissolve in water and include

  • B vitamins

  • Vitamin C

B vitamins include biotin, folate (folic acid), niacin, pantothenic acid, riboflavin (vitamin B2), thiamin (vitamin B1), vitamin B6 (pyridoxine), and vitamin B12 (cobalamins).

Water-soluble vitamins are eliminated in urine and tend to be eliminated from the body more quickly than fat-soluble vitamins. Water-soluble vitamins are more likely to be destroyed when food is stored and prepared. The following can help prevent the loss of these vitamins:

  • Refrigerating fresh produce

  • Storing milk and grains out of strong light

  • Using the cooking water from vegetables to prepare soups

Causes

Disorders that impair the intestine’s absorption of food (called malabsorption disorders) can cause vitamin deficiencies.

Some disorders impair the absorption of fats. These disorders can reduce the absorption of fat-soluble vitamins—A, D, E, and K—and increase the risk of a deficiency. Such disorders include chronic diarrhea, Crohn disease, cystic fibrosis, certain pancreatic disorders, and blockage of the bile ducts.

Some types of weight-loss (bariatric) surgery can also interfere with absorption of vitamins.

Liver disorders and alcoholism can interfere with the processing (metabolism) or storage of vitamins.

In a few people, hereditary disorders impair the way the body handles vitamins and thus cause a deficiency.

If people must be fed intravenously for a long time or if the formula used lacks the needed nutrients, people may develop a vitamin (or mineral) deficiency.

Drugs can also contribute to deficiency of a vitamin. They may interfere with absorption, metabolism, or storage of a vitamin.

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Some Drugs That Cause Vitamin Deficiency

Drug

Vitamin

Alcohol

Folate

Thiamin

Vitamin B6

Antacids

Vitamin B12

Antibiotics, such as isoniazid, tetracycline, and trimethoprim-sulfamethoxazole

B vitamins

Folate

Vitamin K

Anticoagulants, such as warfarin

Vitamin E

Vitamin K

Antiseizure drugs, such as phenytoin and phenobarbital

Biotin

Folate

Vitamin B6

Vitamin D

Vitamin K

Antipsychotic drugs

Riboflavin

Vitamin D

Barbiturates such as phenobarbital

Folate

Riboflavin

Vitamin D

Chemotherapy drugs, such as methotrexate

Folate

Cholestyramine

Vitamin A

Vitamin D

Vitamin E

Vitamin K

Corticosteroids

Vitamin C

Vitamin D

Cycloserine

Vitamin B6

Hydralazine

Vitamin B6

Levodopa

Vitamin B6

Mineral oil (long-term use)

Vitamin A

Vitamin D

Vitamin E

Vitamin K

Metformin

Folate

Vitamin B12

Nitrous oxide (repeated exposure)

Vitamin B12

Oral contraceptives

Folate

Thiamin

Vitamin B6

Penicillamine

Vitamin B6

Phenothiazines

Riboflavin

Primidone

Folate

Vitamin D

Rifampin

Vitamin D

Vitamin K

Sulfasalazine

Folate

Thiazide diuretics

Riboflavin

Triamterene

Folate

Tricyclic antidepressants, such as amitriptyline and imipramine

Riboflavin

Drugs Mentioned In This Article

Generic Name Select Brand Names
AZULFIDINE
No US brand name
CUPRIMINE
ACHROMYCIN V
OTREXUP
SEROMYCIN
DYRENIUM
TOFRANIL
LANIAZID
GLUCOPHAGE
DILANTIN
MYSOLINE
COUMADIN
Levodopa
RIFADIN, RIMACTANE
NIACOR, NIASPAN
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